Method of producing sintered metal bodies



United States Patent Oflice 3,071,463 Patented Jan. 1, 1963 This invention relates to a method of forming sintered metal bodies from powdered metal and more particularly to an. improved method for producing such bodieswhich permits uniform consolidation or compacting of the powdered metal at relatively low pressures preparatory to sintering.

It is the general practice in producing metal parts by powder metallurgical techniques first to compress the loose metal powder in a mold to form a so-called green ingot or compact of the shape of the mold. In a subsequent operation, after the compact is removed from the mold, it is subjected to temperatures sufficiently high to cause sintering of the particles, accompanied by an increase in density and a certain amount of shrinkage of the compact.

The pressing of the metal powder to form the green compacts heretofore has required substantial pressures to obtain the desired unsintered density and the degree of coherence and strength of the pressed masses of particles necessary to permit removal of the compacts from the mold and handling of the compacts prior to the sintering operation. For the majority of metal powders, and in particular those of the refractory type such as, for example, molybdenum, tungsten and stainless steel, the necessary compacting pressures are of the order of to 50 tons or more per square inch of the cross section of the mold or die cavity. To accommodate these pressures it has been necessary to employ relatively massive and costly molds, dies and presses, particularly in the forming of compacts of large size.

Another problem involved in the process heretofore employed has been the non-uniformity of the reduction of the spaces between metal particles throughout the volume of the compact. As may be readily understood, the friction between the particles and between the particles and the walls of the dies or molds tends to prevent uniform transmission of the compacting pressure in all directions. The result is that the particles tend to be deformed to the greatest extent in the direction of application of the compacting force, and those particles remote from the point of application of force tend to be deformed the least, with accompanying lower density of the mass in these latter portions of the compact. Additionally, the green ingots and the final sintered bodies tend to exhibit objectionable directional physical characteristics.

It is, therefore, an object of the present invention to provide a method for producing sintered metal bodies of improved uniformity of physical characteristics.

It is a further object of the invention to provide a method for producing refractory metal bodies from metal powder in which lower pressures than previously have been feasible may be employed to compact the powder into the shape desired, prior to the sintering operation.

In accordance with the method of the present invention, the refractory metal particles, prior to being pressed into the form of the unsintered or green ingot or compact, are suspended in a liquid such as, for example, an aqueous media, and the resulting suspension of metal particles dispersed in the medium is poured into a friable or multi-part, porous or absorbent vessel. The liquid medium is withdrawn or escapes by absorption into the walls of the vessel, leaving a mass of the metal particles supported against the inner surfaces of the vessel. When enough of the liquid medium has been removed from the mass so that it can be separated from the mold at least substantially intact, the mold is taken apart to permit removal of the mass. The mass is then further dried until no more than trace amounts of the liquid medium remain.

The dried mass of particles is then broken up into granules of average size substantially larger than the size of the original metal particles, each granule actually consisting of an agglomeration of the original particles strongly coherent by reason of the manner in which they were settled into the mass of particles from the suspension in the above-mentioned liquid medium. The granules may be produced from the mass in various ways such as, for example, by crushing, milling or screening, or by combinations of such operations.

A quantity of the granules is next placed in a compacting die of the desired shape and pressed to densify the granules into a compact or blank for sintering. It has been found that pressures of the order of 1 or 2 tons per square inch or less are ample in this pressing operation to alford the densities and strengths of the compacts necessary for removal of the compacts from the dies and the handling of the compacts associated with the subsequent sintering step, and to afford the high densities desired in the final sintered blank or part. This is in contrast to the pressures of 10 to 50 tons per square inch or more required heretofore in compacting the metal particles directly, and without first forming agglomerates or multi-particle granules by the suspension and settling steps included in the method of the present invention.

It also has been observed that improved uniformity of density and strength throughout the volume of the compacts is obtained. It is believed probable that this increased uniformity is attributable to the decreased difference between the maximum and minimum effective forces acting on the individual particles throughout the compact. Also related to the improved results in this respect is the fact that a very high density is approached even at very low compacting pressures as will be seen from the data given below in connection with a specific illustration of the improved method. Apparently at the lower pressures involved, there is less frictional force act ing between the walls of the mold and the particles so that the movement of the plunger into the mold can produce a more comparable degree of movement of the particles throughout the mold than heretofore has been attainable.

After the desired pressure has been applied to the powder the compact is ejected from the mold. The compact is then placed in a furnace and heated in a protective atmosphere to cause sintering of the mass. The sintering operation is accomplished in the manner Well known to those skilled in the art and under conditions essentially the same as heretofore have been employed in sintering compacts pressed directly from the original powdered metal. Temperatures below the melting point of the particular metal involved are employed, the particular temperature being related to the time required for the operation, the dimensions of the part being sintered, etc. Typically, for molybdenum, temperatures of from about 1600 C. up to temperatures close to the melting point (2620 C.) may be used and, depending on factors such as those mentioned above, sintering times may be from less than an hour to a day or more.

It has been found that the improved uniformity and density characteristics of the unsintered compacts prepared in accordance with the method of the present invention are reflected also in the final sintered bodies. Thus, as will be more specifically illustrated hereinafter in this specification, sintered bodies of substantially higher density are produced from compacts pressed at the relatively low pressure of tons or less per square inch where the compacts have been prepared in accordance with the present method, as compared with sintered bodies prepared from compacts pressed directly from powdered metal in the manner heretofore employed.

It should be noted that the procedure of suspending the metal particles in a liquid medium and then withdrawing the liquid from the suspension to cause settling of the particles in a close knit and coherent mass is similar to the process of slip casting which has been used extensively in the ceramic art for producing complex shapes for firing. The slip casting technique also has been applied more recently to the manufacture of refractory metal bodies of complex shape. However, in these applications of the slip casting technique to metals, the porous mold used to withdraw the liquid from the suspended particles is of the final shape of the blank to be sintered and care is taken not to break the relatively fragile slip cast compact since it is sintered after drying, without further processing.

Although various liquids, including alcohols, may be employed as the liquid medium in which the metal particles are suspended, water is preferred from the standpoint of cost and the readiness With which it is absorbed by porous vessels of the type particularly suited to this method. In order to keep to a minimum the time required for withdrawal of the liquid medium into the walls of the porous vessel, the ratio of liquid to metal particles should be as small as is consistent with sutficient fluidity of the suspension to permit pouring into the vessel. The precise ratio necessarily depends on various factors such as the size and shape of the metal particles employed and is readily determined in actual practice of the invention.

It has also been found that various metals form suspensions in water which have maximum fluidity at certain ranges of pH values. For example, molybdenum suspensions are of greatest fluidity at a pH of about 6. Suspensions of stainless steel particles generally exhibit maximum fluidity at higher pH values of the order of 10.5. For purposes of the present invention it is preferable to adjust the suspension to the pH value corresponding to the point of highest fluidity before pouring it into the porous vessel. This can be accomplished, for example, by the addition of an alkaline compound such as sodium hydroxide to the suspension in amounts necessary to establish the optimum pH value.

It may also be desirable to add to the liquid medium materials which will aid in stabilizing the suspension, so that an inordinately large increase in the ratio of liquid medium to metal partcles does not occur in the fluid portion of the contents of the porous vessel remaining as the liquid is absorbed by the walls of the vessel. It is of course preferred that the metal particles deposit against the walls of the vessel at no greater rate than the water associated with the particles is absorbed through the walls. In this way maximum uniformity of agglomeration of the particles is assured. It has been found that the addition of small quantities of wetting agents or defiocculent materials such as polyvinyl alcohol or various alginates to the liquid medium afford the desired stabilizing effect.

As an illustration of the advantages realized by employing the method of this invention a number of sintered molbdenum blanks of cylindrical shape were made using a molybdenum powder of the following range of particle sizes.

4 Table I Particle size in microns: Percentage of total powder 0-1 4.66 1-2 18.36 2-3 18-88 3-4 11.67 4-5 4.13 5-6 5.03 6-8 6.29

An aqueous suspension or slip was made consisting of 1 kilogram of the powder of Table I and 200 cubic centimeters of water containing 0.4 gram of ammonium alginate. To maintain the viscosity of the suspension as low as practicable the pH of the suspension was adjusted to about 5 by the addition of a small quantity of sodium hydroxide. The suspension was then poured into a cylindrical water-absorbent plaster mold. After about sixteen hours in the mold sufficient water had been removed into the walls of the mold to permit removal of the mold from the resulting green casting. The casting was dried for 24 hours in a dry box at room temperature. It was then broken up and further granulated or reduced in size by being passed through a 20 mesh screen. The resulting small lumps or granules were substantially larger than the original molybdenum particles and actually consisted of agglomerates of the particles tightly knit together. Typically, these agglomerates may be several microns or larger in diameter.

Sufficient quantities of the lumps or agglomerates of molybdenum particles next were loaded into cylindrical metal mold cavities of 0.403 inch internal diameter to produce compacted cylinders of about 0.25 inch heighth. Compacting pressures of A, l, 2, 5, 10 and 20 tons per square inch of cross-sectional area of the compacted cylinders were employed.

Another portion of the powdered molybdenum of Table I was compacted into small cylinders using the mold cavities referred to in the preceding paragraphs. The same series of compacting. pressures was employed, but the powder, prior to being compacted, was not subjected to the slip casting and granulating operations of the invention.

From the weights and dimensions of the two groups of compacted cylinders, the densities of the cylinders compacted at various pressures were as shown in Table II, below:

Table II Cylinders Density of (grams/cu. compacted cm.) from Compacting pressure, tons/sq. in. from tmpowder of treated pow- Table I after der of slip casting Table I and screening treatment It will be noted in Table II that no values for the density of the compacts pressed from the untreated. powder at pressures of A and 1 ton per square inch are given. Actually, at these low pressures the untreated powder could not be consolidated into cylinders of sufficient coherence and density to permit removal intact from the mold cavity. Hence no density measurements could be made at these low pressures. On the contrary, it is readily seen from the data in the last column of Table II that even at the low compacting pressures of and 1 ton per square. inch, the density of the compacts, produced in. accordance with the methodof the invention is of the same order of magnitude as the density of the compacts molded directly from the molybdenum powder at the substantially higher pressure of 20 tons per square inch. It is further evident that at equivalent compacting pressures throughout the range employed a substantially higher density compact is obtained if the powder is first slip cast and broken up into agglomerates of the original particles.

Cylindrical compacts made by both procedures as delcribed above were sintered for 5 hours at 1700 C. The densities were determined from their dimensions and weight and are shown in Table III.

From Table III it is seen that the sintered cylinders made from the molybdenum powder which was slip cast and screened into granules prior to compacting were of substantially greater density than the sintered cylinders from the compacted original molybdenum powder. This difference in density is particularly noteworthy with respect to the sintered cylinders made from the compacts produced at lower pressures. Although no sinterable cylinders of the original molybdenum powder compacted at pressures of 4 and 1 ton per square inch could be obtained, comparison can be made between the densities of 6.43 and 9.25 per cubic centimeter of the cylinders produced from compacts pressed at 2 tons per square inch, the cylinders produced from molybdenum powder treated in accordance with the present invention being of the higher density.

To determine whether the increased density of the green compacts might have been wholly or in part attributable to the presence of the small amount of ammonium a-lginate added to stabilize the suspension of original molybdenum powder, two additional groups of cylindrical compacts were made from the powder of Table I. In the preparation of the one additional group of compacts, a suspension or slip of the molybdenum powder of the composition speci-fically described above, containing 1 kilogram of molybdenum powder, 200 cubic centimeters of Water, 0.4 gram of ammonium alginate, and sufficient sodium hydroxide to maintain the suspension at a pH of 5, was first prepared. This suspension was then slip cast in a plaster mold, dried, broken up into lumps and granulated through a 20 mesh screen as has been previously described in the specification. The granulated material was then fired at 400 C. for four hours to remove the ammonium alginate. Samples of this granulated material were compacted at various pressures in the cylindrical mold described above and densities of the resulting compacts were measured. The results of this test are set forth in column 1 of Table IV.

A second additional sample of molybdenum powder was prepared by thoroughly dry blending 0.04 percent of ammonium alginate into a batch of the molybdenum powder of Table I. Samples of the resulting powder were compacted in the cylindrical molds at various pressures and the densities of the green compacts were then measured. The results of these measurements are shown in column 2 of Table IV below. No density Values are given for compacts molded at pressures of A and 1 ton per square inch for the reason that the compacts were not sufficiently consolidated at these pressures to permit removal from the mold cavity.

By comparison of the density values given in column 2 of Table IV with the density values given in Table II for compacts formed directly from the original molybdenum powder it can be seen that the presence of ammonium alginate in the powder did not result in compacts of generally greater density nor did it make possible the forming of compacts at very low pressures.

Furthermore, by comparing column 1 of Table IV with the density values given in Table II for compacts formed from molybdenum powder which had been sub jected to the preliminary slip casting and screening treatment, it is evident that removal of the ammonium alginate from the material did not detract appreciably from the high densities obtained. The slightly lower densities shown in Table IV were believed attributable to the slight disintegration of the granules of molybdenum powder in the handling associated with firing the material to remove the ammonium alginate.

Although the method of the invention has been described above with particular reference to its application to molybdenum powders, it is applicable to a wide variety of metal powders, and affords particular advantages in the production of sintered bodies of refractory metals including, but not limited to, tungsten and stainless steels as well as molybdenum.

What is claimed is: v

1. The method of producing a sintered metal body from metal particles which comprises slip casting a quantity of said metal particles to form a coherent mass thereof, breaking up said mass into agglomerates of said particles, pressing a quantity of said agglomerates to form a compact thereof and sintering said compact to form said metal body.

2. The method of producing sintered metal bodies from metal particles which comprises dispersing a quantity of the metal particles in a liquid medium, removing the liquid medium from the particles to cause agglomeration of said particles into a coherent mass thereof, breaking up said mass into granules, each of said granules consisting of a plurality of said particles, pressing quantities of said granules to form compacts thereof and sintering said compacts to form said bodies.

3. The method of producing a sintered molybdenum body which comprises suspending molybdenum powder in an aqueous medium, pouring the resulting suspension into a vessel having porous walls, thereby to form a mass of said powder in said vessel as the aqueous medium escapes into the vessel walls, drying and disintegrating said mass into granules of average size larger than the average particle size of said powder, pressing a quantity of said granules in a mold to form a compacted blank and thereafter sintering said blank.

4. In the process of producing a refractory metal body in which a compacted blank of the metal powder is sintered to form said body, the steps which include mixing a quantity of the powder with water, removing the water from the mixture by maintaining the mixture in contact with a porous surface capable of absorbing the water, thereby to deposit a coherent mass of the powder against said surface, removing the mass from said surface, subdividing said mass into agglomerates of the powder, and compressing a quantity of the agglomerates into the form of said compacted blank.

into granules of substantially greater size than the parti- 21839819 cles of said powder, pressing a quantity of the granules. in a mold to form a compact, and heating the compact to a temperature of between about 1600 C. and the melting point of molybdenum to cause sintering together of the metal particles.

References Cited in the file of this patent UNITED STATES PATENTS 2,545,438 Stumback Mar. 20, 1951 2,744,011 Samuel et a1. May 1, 1956 Platte June 24, 1958 

1. THE METHOD OF PRODUCING A SINTERED METAL BODY FROM METAL PARTICLES WHICH COMPRISES SLIP CASTING A QUANTITY OF SAID METAL PARTICLES TO FORM A COHERENT MASS THEREOF, BREAKING UP SAID MASS INTO AGGLOMERATES OF SAID PARTICLES, PRESSING A QUANTITY OF SAID AGGLOMERATES TO FORM A COMPACT THEREOF AND SINTERING SAID COMPACT TO FORM SAID METAL BODY. 